This invention relates to power trains in general and, in particular, to a vehicle power train of the type having a torque converter equipped with a lockup clutch, and a transmission having a plurality of shift clutches for selecting a desired speed setting and a desired traveling direction. More particularly the invention pertains to a hydraulic system for controlling the operation of the lockup clutch in such a power train. The power train with the control system of this invention has particular utility in conjunction with some off-highway self-propelled work machines.
In some vehicle power trains of the type under consideration the lockup clutch is hydraulically engaged and disengaged under control by the vehicle operator. When engaged, the lockup clutch changes the power train from hydrodynamic to mechanical drive, bypassing the torque converter and mechanically coupling the vehicle engine to the transmission. A problem arises if, with the lockup clutch held engaged, the transmission is shifted to a different speed setting by actuation of one of the shift clutches. Since then the transmission is in direct mechanical connection with the engine, its shifting gives rise to a violent shock and causes sudden exertion of great torque on the engaged shift clutch, thus inviting rapid wear of the shift clutches.